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Never ceases to amaze me how many hvac techs have a total misunderstanding of ac power, and how incorrect opinions are foisted off as wisdom, must be a gross error in some hvac text somewhere? .

FWIW:
1. good news - everyone is correct in that the caps should be rated at 440 Vac for good reliability.
2. bad news - not a single poster seems to understand ac power flow. The 389 or 400 V across the cap IS NOT generated by the start or run winding. The voltage across the cap plus the voltage across the run winding is exactly equal to the line voltage at EVERY specific point in time. When you hook up your digital voltmeter, you are probably reading 1.11 time the average voltage ( any cheap meter, even a Fluke 77 reads 1.1 times average vs true rms). If you hook up a scope (e.g. Fluke 123 or equivalent) across the cap and run winding, you would see that at times the volts of the sine wave across the cap is negative and across the winding is positive and both are higher peaks and rms than line voltage - read up about leading and lagging currents in capacitors and inductors before blathering about back emf, etc. and about resonance and voltage amplification thru resonance, etc. etc.

Junkhound, do yourself a favor. Before blasting people here over giving incorrect information, try researching some about how AC motors work, because you are dead wrong.

Back EMF is real, and the other posters are correct. In an induction motor with a run and start winding, there is an induced voltage potential between the run and start windings that exeeds the input voltage to the motor.
This is the same voltage that energizes the coil of a potential relay for a start kit, and is the voltage the capacitor must have a higher rating than.

1. never said there was no such thing as back emf, but that is not what causes capacitor voltage higher than line voltage. A cap and inductor of any type (not even a motor) can give higher voltages across the cap than line voltage.....

2. interesting how a bunch of internet links can be re-interpreted to suit preconceived notions.

Hook up a 4 channel scope to a motor and them come back and make comments.

Junkhound, the links were just random Google stuff. Your origonal comment is still wrong.
I have plenty of hard copy text here, I'll see if I can find something to link that is more relevent to PSC motors when I get home tonight.

Junkhound, I have some test data in front of me that says we have a unit running @ 230 VAC with a capacitor voltage of 336.3 VAC (.5% instruments with tracable calibration). If your theory holds true, then the capacitor voltage should be 325 VAC, which is peak voltage at a measured voltage of 230 VAC RMS.

Unless I misunderstood something you said, which is certainly possible.

....then the capacitor voltage should be 325 VAC, which is peak voltage at a measured voltage of 230 VAC RMS....

You misunderstood, the capacitor voltage has nothing directely to do with 1.414 times the rms value of the line voltage. The capacitor voltage is the capacitor reactance times the run circuit current. The run circuit current is the (vector 'sum' of line voltage minus back emf) divided by the run circuit vector impedance. The vector impedance is the vector (NOT arithmetic) sum of the the winding impedance and the capacitor impedance. Since the vector sum of the inductive and capacitive impedances is often LESS than the impedance of either the run winding or the capacitor, the voltage across the capacitor is higher than the line voltage.

The higher voltage across the cap is normal, what is not good in the original post is that the manufacturer chose to save a few pennies at the expense of reliability and not do a worst case analysis of the run circuit current and capacitor voltage.

BTW, had a call-back on a Goodman condensor last month, the scroll rubber feet are held by glorified sheet metal screw which came loose, causing low frequency vibration of the compressor, a few more months and a tube could have broken from fatigue.

BTW, had a call-back on a Goodman condensor last month, the scroll rubber feet are held by glorified sheet metal screw which came loose, causing low frequency vibration of the compressor, a few more months and a tube could have broken from fatigue.

A cap and inductor of any type (not even a motor) can give higher voltages across the cap than line voltage.....

this my origional point
yes when you look at voltage on an ocilliscope it peaks higher than 240 line voltage. meters are buit to read what is called root mean voltage. which is lower than the peak of the sine wave.

for thouse of you that are old enough to remember the old tv show " we are taking controll of your horizontzl we are taking controll of your vertical" the wave you saw was a sine wave hehehehehe.